Leach column and method for metal recovery

ABSTRACT

A leach column ( 10 ) and method for the separation and recovery of metals from mixtures. The column ( 10 ) is adapted to be electrically charged to enhance metal recovery. Particularly, a pair of electrical connections ( 20 ), ( 22 ) are positioned in the column ( 10 ) to create an electrical charge in the presence of a leaching solution ( 18 ) and a metal-containing mixture ( 26 ) to enhance leaching of the metal from the mixture. In an exemplary embodiment, the leach column ( 10 ) is used for the recovery of platinum from platinum-containing coatings on jet engine components.

BACKGROUND OF THE INVENTION

[0001] I. Field of the Invention

[0002] The present invention relates to the recovery of a metal from a mixture, and more particularly, to a leach column for use in recovering the metal.

[0003] II. Description of the Prior Art

[0004] Metal-containing mixtures, such as ore (an aggregate or mixture of metals), contain one or more metals which are useful and desirable in everyday life. Mining for specific metals is a big business and large amounts of money are invested everyday for the recovery of desirable metals. However, to render these metals useful, the metal-containing mixtures, whether mined or otherwise, must be refined to reclaim and recover the desired metal. Refining processes may be costly and add to the consumer's cost for the metal. Mixtures containing inexpensive metals, such as copper, zinc, nickel and the like, are often discarded as waste or scrap and not recovered. In particular, where they are present in the mixture in small quantities, the costs of recovering the metal generally outweighs the value of the metal recovered. However, many discarded mixtures contain such metals in quantities sufficient to warrant an attempt to recover the metal. Aside from the cost of the metal, disposal of such mixtures may present environmental issues and associated landfill problems. It is therefore desirable to recover less expensive metals for recycling and reuse.

[0005] It is also desirable to recover more valuable metals from mixtures. These metals are in demand due to their precious nature and/or low supply on the open market. For example, precious metals such as platinum, gold and silver are commonly sought after for use in jewelry, and platinum, aluminum, chromium, and palladium, are highly desired by commercial customers for commercial needs. Platinum, in particular, is commonly used to strengthen and repair jet engine components through the application of platinum-containing alloy coatings. Jet engine components, manufactured to have a long working life and to be exposed to harsh conditions, are typically coated to provide added corrosion resistance and strength. Coatings on components such as turbine blades, vanes, and/or shrouds, for example, may contain small amounts of platinum in the form of a platinum-aluminide alloy. This coating acts as a protective barrier to provide resistance to the high temperatures, air velocity, and other harsh conditions to which these engine components are exposed during normal operation and flight.

[0006] During normal operational maintenance and times of repair, the jet engine component is often removed and stripped of its outer coating in order to perform the repair and/or to restore the component to the original manufacturer specifications. After the repair has been completed, a fresh platinum aluminide coating is deposited, typically by electroplating methods, on the component surface. The platinum aluminide coating is a mixture containing desirable platinum, typically present in small quantities. For example, on a GE CF6-80C2 stage 1 high pressure turbine blade, there is approximately 1 gram of platinum in the coating. On smaller turbine blades, the amount of platinum is generally in the range of 0.5-0.8 grams, whereas on larger blades, there is up to about 1.5 grams of platinum. During normal engine operation, a portion of this platinum may typically evaporate or be removed by abrasive action from particles entrained in the gas flow through the engine. However, a majority of the platinum remains in the mixture and is present when the coating is stripped.

[0007] Various methods of stripping these platinum-containing coatings have been utilized. For example, stripping solutions involving the use of corrosive acids such as nitric acid, sulfuric acid, phosphoric acid, hydrochloric acid, and mixtures of these acids have been used. These acids work as electrolytes in a corrosion reaction to remove or strip the coatings from the underlying metal. While using such corrosive agents, it is desirable to avoid damage to the underlying substrate during the stripping process. Thus, the stripping solutions generally dissolve a portion of the coating, but do not completely remove the coating. Instead, a significant portion of the coating remains undissolved and in the form of smut or residue adhered to the surface of the component. As such, the stripping solution generally contains only minute quantities of coating constituents including platinum, with the majority of the coating constituents remaining on the surface as the smut or residue. After stripping, the component is rinsed to remove residual acid and loose smut, and filters are used to capture any platinum-containing particles in the removed smut or stripping solution.

[0008] The smut is typically present in the form of a thin, loosely adhered film which does not come off simply by rinsing. Various methods have been developed to remove the smut from the surface of the component. For example, one method is to grit blast the stripped surface with sand particles or aluminum oxide particles of various size. After grit blasting, the particles removed from the component are mixed in with the sand grit particles, and this particulate mixture is generally disposed of as waste, including the valuable platinum and platinum-bearing particles.

[0009] The residue or smut is generally a particulate material comprising platinum or platinum-bearing alloys, aluminum oxides, and heavy metal oxides of various elements from the substrate materials as part of the coating constituent. Despite being present in small quantities, the platinum is worth recovering from the residue if the concentration of the platinum in the recovering solution can reach a minimum of 0.5% by weight of the mixture. Traditionally, solutions used to strip and recover platinum from the residue have contained platinum in lower percentages and, therefore, typically have been discarded and disposed of in landfills as waste. Separation and recovery of the platinum from such residue has been time consuming with poor recovery yields and is often considered to be a worthless effort. Such waste generated from aircraft repair facilities is generally regarded as hazardous and, therefore, must be disposed of in a hazardous waste landfill. Recovery and reclamation of as much platinum as possible from these coatings not only saves costs, due to its valuable nature and the ability to recycle and reprocess the platinum into future coatings and/or other uses, but may also allow the waste generated from repairs to be downgraded to normal waste and disposed of in a normal landfill, thereby eliminating the added requirements necessary for disposal in a hazardous waste landfill.

[0010] There are many proposed methods for recovering desirable metals from mixtures. Some methods involve washing and rinsing the mixtures with solutions to wash away the contaminated waste. Hydrometallurgical methods involve the use of acidic solutions to oxidize metals to their corresponding salts for isolation and subsequent processing to recover pure metal. Further, chromatographic methods have been used to isolate metals on the basis of size, shape, and other physical and chemical properties of the metal. However, these methods yield poor recovery and are generally limited to recovering metals from small quantities of mixtures.

[0011] Another proposed method is leaching. Leaching has been proposed for separating and recovering metals from large volumes as well as small volumes of mixtures. Leaching is generally a process whereby a desired metal is extracted or leached from a metal-containing mixture or compound. The material is generally exposed to a leaching solution which dissolves and extracts the metal. Leaching methods have been modified to improve recovery of metals. For example, leaching systems utilizing acid, ammonia, and other suitable reagents have been agitated to improve metal extraction. Leaching while exerting pressure onto metal-containing mixtures has been tried. Heap leaching in columns is gaining in popularity.

[0012] The leaching process generally occurs within a leach column or on a bed of inert material. Generally, the leach column is packed by loading the material to be leached into the column, or the material is placed on an impermeable pad or a bed of inert material, such as sand, within the column. A leaching solution is applied to the material at the top of the column and allowed to percolate through the material in the column to extract metal. The metal dissolves into the leaching solution, thereby being selectively removed from the material. The metal-containing leach solution or leachate is generally collected by a drainage system at the base of the column and channeled to a leachate collection reservoir. The collected leachate may then be processed to recover the metal, such as by smelting or other conventional methods. The leach solution remaining after metal recovery is generally “clean” and may be recycled back into the leach column.

[0013] These proposed metal recovery methods, however, are not very efficient relative to the time necessary for recovery, and afford only a small percentage of the desired metal contained in the mixture. Consequently, much of the metal is left in the material and not recovered. This unrecovered portion is generally lost as waste. Thus, the amount of metal recovered from such materials, particularly with materials containing smaller quantities of desired metals, is typically not worth the time and effort spent in recovering it.

[0014] Thus, there is a need to recover a significant portion of metals present in metal-containing materials, particularly from materials containing small quantities of the metal. Further, there is a need to recover platinum from the coatings on jet engine components such as turbine blades. Still further, there is a need to recover the metal efficiently and economically from a cost and time perspective.

SUMMARY OF THE INVENTION

[0015] The present invention provides apparatus and methods for recovering metals, such as platinum, gold, silver, palladium, nickel, copper, cadmium, rhodium, and zinc, among others, from particulate materials while addressing and overcoming some of the weaknesses and drawbacks of the metal recovery techniques of the prior art. To this end, there is provided a leach column within which a metal-containing particulate material is placed for leaching and separation of a desired metal. The leach column is adapted to induce an electric current through a leaching solution in the column during the leaching process. An electric current raises the extraction potential of the leaching solution by electrically charging the metal, thereby enhancing dissolution of the metal into the leaching solution. As a result, a greater quantity of the metal is leached and separated from the metal-containing particulate material.

[0016] To induce the electric current, the leach column may include a pair of electrical connections, such as a pair of electrodes, spaced apart at desired locations in the column. Advantageously, the electrodes are placed at opposing ends of the column. The electrical connections or electrodes should be made of suitable materials, such as a corrosion resistant alloy material or a carbonaceous material, for efficient conduction of electricity. In one embodiment of the present invention, the electrodes comprise graphite. The electrical connections are attached to an external source of electricity, such as a DC source, for inducing an electric current within the column. The leaching solution serves as an electrolyte between the pair of electrical connections so as to initiate the electric current.

[0017] The metal-containing particulate material, generally a compound or a mixture, is placed within one or more leach columns to begin the leaching process. A leaching solution capable of leaching metals from the mixture is allowed to pass through the material. For metals such as platinum, gold and the like, this solution may contain one or more mineral acids, such as nitric acid, sulfuric acid, hydrofluoric acid, hydrochloric acid, phosphoric acid, sulfamic acid, citric acid, oxalic acid, and/or peroxide. While the leaching solution percolates through the material, either by gravity or with the aid of a pump, an electric current is passed through the column between the electrical connections. Leaching is enhanced with the electric current and the leachate is collected from the column outlet. The material may be leached several times over with leaching solution over a period of several days. The collected leachate may then be further processed for recovery of the leached metals and/or refined for reuse. For example, the leachate may be distilled for recovery of “metal-free” leaching solution that may be recycled back into the leach column.

[0018] The present invention is useful in separating desirable and precious metals, such as platinum, gold, silver, and palladium from materials containing small quantities of the metal. For example, the leach column is useful for recovering platinum from platinum-containing coatings stripped from jet engine components, including turbine blades, vanes, or shrouds, in a platinum recovery system in accordance with the present invention. The platinum recovery system may further include other desirable apparatus, such as a distillation set-up for recovery of leaching solution, tanks for the collection of the leaching solution and the leachate, pumps, and control devices for controlling the leaching process. The exemplary platinum recovery system allows recovery of greater amounts of platinum from the coating-residue than methods of the prior art.

[0019] By virtue of the foregoing, there is thus provided a leaching column and method for the recovery of desirable metals, such as platinum, from a compound or mixture. The present invention renders economically feasible the recovery of metals from mixtures which may otherwise have been discarded as waste. Thus, the present invention provides a savings in terms of the conservation of metals and the costs and time associated with recovery thereof. These and other benefits and advantages of the present inventions shall be made apparent from the accompanying drawings and the detailed description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings (not to scale), which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general description of the invention given above and the detailed description of the drawings given below, serve to explain the principles of the present invention.

[0021]FIG. 1 is a perspective view of an exemplary leach column having two electrodes in accordance with one embodiment of the present invention;

[0022]FIG. 2 is a top view of one electrode positioned in the leach column illustrated in FIG. 1;

[0023]FIG. 3A is a perspective cut-away view of the leach column illustrated in FIG. 1 having a particulate mixture and leaching solution in an exemplary embodiment of the leaching method;

[0024]FIG. 3B is a perspective cut-away view of the leach column illustrated in FIG. 1 having a particulate mixture and leaching solution in a second exemplary embodiment of the leaching method;

[0025]FIG. 3C is a perspective cut-away view of the leach column illustrated in FIG. 1 having a particulate mixture and leaching solution in a third exemplary embodiment of the leaching method;

[0026]FIG. 4 is a cut-away view of a base in the lower end of the leaching column illustrated in FIG. 1;

[0027]FIG. 5 is a bottom view of the base illustrated in FIG. 4;

[0028]FIG. 6 is a perspective view of a first exemplary metal recovery system in accordance with the principles of the present invention; and

[0029]FIG. 7 is a block diagram of the metal recovery system illustrated in FIG. 6.

DETAILED DESCRIPTION OF THE DRAWINGS

[0030] The present invention provides a leach column adapted to induce an electric current therein to enhance the leaching and recovery of metals from a metal-containing material. The present invention may be used to recover metals from various types of “materials”. For example, the material may be a particulate mixture having metal particles therein or a compound comprising a metal alloy having the desired metal in combination with an alloy. To this end, the term “metal-containing material” is intended to have broad meaning, and is not limited to a particulate mixture. Examples of the metals that may be recovered with the leach column include, without limitation, platinum, gold, silver, palladium, nickel, copper, cadmium, rhodium, and zinc. Leaching in the presence of an electric current improves recovery of metals from metal-containing materials over prior art leaching methods. The term “leaching”, as used herein, is intended to refer to the extraction of a metal from a particulate material by a solution into the solution. Extraction results when the metal partially or fully dissolves in the solution thereby separating the metal from the particulate material. The term “leaching solution”, as used herein, is intended to refer to the solution capable of dissolving or extracting the metal from the particulate material. The leaching solution is a mobile phase in that it moves or passes through the particulate material in the column to cause a separation of components in the material. The term “leachate”, as used herein, is intended to refer to the metal-containing leaching solution, i.e., the solution that passes out of the leach column and contains the leached material. The column may be pre-packed with an bed of inert material, such as sand, to form a substrate within the column on which the material to be leached is placed. Alternatively, the particulate mixture to be leached may already include the inert material. For example, sand or other particulate material used for grit-blasting surfaces of jet engine components to remove coating residue is a suitable inert material, and forms part of a particulate mixture with the residue particles.

[0031] With reference to FIG. 1, there is shown, in perspective view, an exemplary leach column 10 comprising a tube 12 shaped as a cylinder having a top end 14 and a bottom end 16. Tube 12 may be generally formed of conventional materials, such as a plastic or other polymeric-type material, including polypropylene, polyvinylchloride (PVC) and the like, or glass, which are generally inert and non-reactive to leaching solutions 18 (FIGS. 3A-C) used therein. It should be understood that useful leaching solutions generally include corrosive acids or caustic bases that may adversely react with some materials such that the tube 12 material should be selected to resist such reactions. Column 10 may be any desired length and diameter. For example, a suitable column 10 is a plastic (PVC) column with a 4-inch inner diameter and an 8-foot length. Such a column is capable of holding greater than 100 lbs of material and would be useful for heap leaching large amounts of particulate materials. A long column 10 provides longer leaching paths, increased separation, and can accommodate large amounts of metal-containing material. But a long column 10 also results in longer metal recovery times, increased leaching solution and more inert bed materials (where used). The thickness of the tube 12 may play a role in level of pressure that column 10 may withstand be during the leaching process. For example, where leaching is aided with use of a pump (not shown), tube 12 should be of a thickness to withstand pressures exerted in column 10 by the pump.

[0032] Leach column 10 is adapted to induce an electric current, thereby providing advantages in the separation and recovery of metals from metal-containing materials over the metal recovery techniques of the prior art. To this end, tube 12 includes a pair of electrical connections at spaced apart locations and configured to induce the electric current therebetween in the presence of a leaching solution 18 (FIGS. 3A-C). As shown, a first electrode 20 and a second electrode 22 are illustrated as the pair of electrical connections. While FIG. 1 illustrates an electrode as the electrical connection, the present invention is not so limited, and the electrical connection may be any connection capable of serving as either an anode or a cathode for the purpose of conducting current within an electrical circuit. The electric circuit is complete when the first electrode 20 and the second electrode 22 are in contact with a leaching solution 18, and both electrodes are connected to an external electrical source 24.

[0033] The external source 24 illustrated in FIG. 1 is a battery. However, the present invention is not so limited and external source 24 may be any source of electricity suitable to provide an AC or DC current for passage through the leach column 10. For example, the external source 24 may be an electric generator. By way of example, for an 8 foot column, source 24 may generate a DC voltage in the range of about 2 volts to about 19 volts, which will allow a current through the leach column 10 which, at any given time will generally be proportional to the amount of metals dissolved in the leaching solution. However, depending on the resistance in the circuit, more or less DC voltage may be necessary to provide a current of sufficient amperage, such as about 0.5 amps, through the leach column 10 to enhance leaching of the metal. In one embodiment of the present invention, the source 24 is a DC power rectifier, Model No. RC-1010, manufactured by PDI, Tucson, Ariz. An AC current will also be effective to enhance leaching of metals due of the difficulty of the metal(s) to electroplate the electrodes in the presence of the AC current. The leaching solution 18 between the first and the second electrodes 20 and 22 and in contact therewith, provides the electrolyte necessary for the conduction of the electric current.

[0034] Either of the first electrode 20 or second electrode 22 may serve as an anode while the other serves as a cathode for transmission of the current. First electrode 20 and a second electrode 22 are spaced apart at desired locations, advantageously at opposing ends 14 and 16 of tube 12 as illustrated. Placement of electrodes 20 and 22 at opposing ends of tube 12 maximizes the column length through which the electric charge flows. In one embodiment of the present invention, the, first electrode 20 is placed at the top end 14 of tube 12 while the second electrode 22 is placed at the bottom end 16 of the tube 12. However, the locations illustrated are exemplary and the first electrode 20 and second electrode 22 may be positioned at any desired locations within column 10.

[0035] With reference to FIG. 2, there is shown, in a top view, the exemplary first electrode 20 illustrated in FIG. 1. As shown, first electrode 20 is shaped as a “T” and has an electrode terminal head 30 which is positioned inside column 10 at the top end 14 and abuts the inner wall 32 of column 10. The terminal head 30 spans a portion of the inner surface 32 of column 10 to optimize electric charge therein. First electrode 20 also comprises a tail portion 34, spanning tangentially from the outer circular edge of column 10, external to column 10, and providing a means to attach the external source of electricity 24. The external source 24 may be attached via a circular hole 36 into the tail portion 34 of first electrode 20. Advantageously, the second electrode 22 is identical to the first electrode 20 in shape, design, and mode of connection to the external source 24. More advantageously, the first electrode 20 and second electrode 22 comprise suitable electrically conductive materials, such as a corrosion resistant alloy material. Examples of corrosion resistant alloy materials include, without limitation, nickel-base alloys, cobalt-base alloys and nickel-chromium-molybdenum alloys, such as HASTELLOY® C-276. Additional suitable materials include, without limitation, platinum, palladium, niobium-expanded mesh coated with platinum, such as DCX 125 (125 μ-inch platinum over double-clad expanded niobium) (available commercially from Vincent Metals, Canonchet, R.I.), platinized titanium (titanium (expanded mesh or non-mesh) plated with platinum, then heat treated to diffuse/disperse the platinum onto and into the titanium). Carbonaceous materials are also suitable materials for the first and second electrodes 20 and 22. However, electrodes 20 and 22 should not be formed of or comprise materials that may be leached by the selected leaching solution. For example, a platinum electrode or platinum-containing electrode should not be used where the metal to be recovered from the particulate material is platinum. In one embodiment of the present invention, the first and second 20 and 22 electrodes comprise graphite. Examples of suitable graphite electrodes are available from POCO Graphite of Decatur, Tex. under product designation AFX-5Q®. Graphite is relatively inexpensive and renders the leach column cost effective.

[0036] With reference to FIGS. 3A-C, there is shown, in multiple perspective cutaway views, the exemplary leach column 10 of FIG. 1, but having leaching solution 18, metal-containing particulate material 26 and inert material 28 contained therein. FIGS. 3A-C depict three exemplary embodiments of a particulate mixture that includes metal-containing particulate material 26, placed within the leach column 10, for leaching of a metal therefrom by passage of a leaching solution 18 therethrough. In the embodiment depicted in FIG. 3A, the metal-containing particulate material 26 is generally placed at the top end 14 of column 10 onto a bed or substrate of inert material 28, which may be mineral particles such as sand, pre-packed within the column 10. The particulate material 26 is initially separate from the inert material 28. In the embodiment depicted in FIG. 3B, the particulate material 26 is in a particulate mixture with an inert material 28, within the particulate material 26 dispersed throughout the inert material 28. As shown, the mixture of particulate material 26 and inert material 28 completely fills column 10, and therefore, column 10 need not be pre-packed with a substrate of inert material 28. In the embodiment illustrated in FIG. 3C, column 10 is pre-packed with a substrate of inert material 28 and a mixture of particulate material 26 and inert material 28 is loaded onto the pre-packed substrate of inert material 28. While the description herein makes reference to sand as the inert material 28 in column 10, sand is merely an example of suitable materials that are inert to the leaching solution 18. Other materials such as gels, resins, silica and the like may also be used as an inert material 28 in column 10. When column 10 is not in use, the top end 14 and bottom end 16 of column 10 may be capped for protection of any inert material 28 contained therein, and removed while column 10 is in use, or when the inert material 28 needs to be replaced or replenished.

[0037] In the manufacture and preparation of leach columns 10 having first and second electrodes 20 and 22 inserted therein, care should be taken so as to avoid any cracks, empty spaces or gaps between the column wall and the electrode that might allow the leaching solution to leak out of the column and onto its outside surface or into the work environment. Thus, properly sealing the electrode mounts or insertions into the leach column 10 is desired.

[0038] With reference to FIG. 4, there is shown, in cross-sectional view, the bottom end 16 of leach column 10 illustrated in FIG. 1. Bottom end 16 includes an exemplary base 38 which may be used to stop, plug or generally close bottom end 16 of column 10. Base 38 generally provides a barrier against the passage of sand or other inert material 28 out of column 10. Base 38 is inserted into the bottom end 16 of column 10 and held in place via the use of an O-ring 40 as shown. Other conventional means to attach base 38 to column 10 are also suitable. Advantageously, base 38 is attached in a manner to withstand pressures exerted in column 10 during the leaching process, particularly where leaching is aided by use of a pump (not shown).

[0039] With reference to FIG. 5, base 38 may be circular in shape to match the diameter of column 10 and may comprise a lip 42. However, it may be appreciated that column 10 and base 38 are not limited to any particular shape. Lip 42 is useful for insertion and removal of base 38 with respect to column 10. Base 38 further includes perforations or holes 44 through which the leaching solution 18 (leachate) can pass. As shown, the holes 44 may be present only on one side of base 38 where the column 10 is placed at an angle during the leaching process (FIG. 6). However, the number of holes 44, location of the holes 44, and the diameter of each hole 44 may vary as desired depending on the desired flow rate of the leaching solution 18 through column 10. Referring again to FIG. 2, the holes 44 may be located equidistant around the circumference of the base 38 for facilitating an even flow of the leaching solution out of the column 10. It should be understood that the diameter of holes 44 will dictate whether or not the inert material 28 will also seep through the base 38 of column 10. Generally, the diameter of holes 44 should be smaller than the diameter of the majority of the particles comprising the inert material 28. For example, the hole 44 may be ⅛th of an inch in diameter to allow passage of the leaching solution 18 (leachate) therethrough while preventing passage of a majority of the inert material 28.

[0040] One or more leach columns 10 are useful in recovering precious metals, such as platinum, from mixtures or residues containing the platinum, even in small quantities. To this end, and with reference to FIG. 6, there is shown, in perspective view, an exemplary metal recovery system 50 in accordance with the present invention. As shown, metal recovery system 50 comprises three leach columns 60 parallel to one another and supported by column supports 62. Each column 60 has a bottom end 64 and a top end 66. A particulate mixture with metal-containing particles 68 and inert material 76 is placed within column 60 for leaching of a metal therefrom. The top ends 66 of columns 60 are connected (not shown) to a supply of leaching solution 70 from a leaching solution reservoir 72. The reservoir 72 is advantageously connected to the top ends 66 by any suitable means necessary to provide a flow, advantageously a continuous flow, of leaching solution 70 into the top end 66 of each column 60 for leaching of metal. The reservoir 72 may be heated by a heater 73 for providing warm or hot leaching solution 70 to column 60. Leaching of metals is generally enhanced as the temperature of the leaching solution 70 increases. However, safety may be compromised with a very hot leaching solution 70. The bottom end 64 of each column 60 terminates in a collection box, such as a sand box 74, such that any inert material 76, such as sand, passing through the holes 77 in the base 78 of each column 60 will collect in the sand box 74. In the exemplary embodiment illustrated, the inert material 76 of column 60 is sand and is present as a particulate mixture with metal-containing particles 68 in the columns 60. Attached to the sand box 74 are one or more washing stations or washers 80 in which the sand 76 (or other inert material 76) may be washed free of leaching solution 70 and any leached metal contained in the leachate. The sand 76 may then be dried in a dryer 82 so that it may be reclassified for further refinement and/or use. For example, sand 76 having a particle size suitable for grit blasting may be reused as grit for such purpose. The remaining sand 76, typically having smaller particle sizes, may be sold as a “flour” and used for other purposes, or disposed of in a normal non-hazardous waste landfill. If the inert material 76 removed from the column(s) 60 during leaching is not efficiently cleaned and effectively recovered, it has to be disposed of as hazardous waste due to contamination with hazardous leaching chemicals.

[0041] The leaching solution 70 passing out of each column 60 is referred to as “leachate”, and contains metals and impurities leached from the particulate mixture in minute quantities. Referring to FIG. 7, a block diagram view of the exemplary metal recovery system illustrated in FIG. 6, the leachate is channeled from the sand box 74 into a leachate tank 84, optionally through a washer 80, as described herein. The leachate will contain metals dissolved from the original metal-containing particulate material 68. This leachate may be further refined or processed in order to recover the metals separated from particulate material 68. For instance, a distillation station 86 may be utilized to separate the leaching solution 70 from the leached metals contained therein. The distillate (leaching solution 70) collected may be recycled back into the top ends 66 of columns 60 or back into the leaching solution reservoir 72. For example, a suitable distillation station 86 for distillation of acid-comprising leaching solutions may include a Barnstead Distillation Mega-Pure Glass Still System, Model A440697-MP6A, which is capable of distilling acid at a rate of 6 liters per hour capacity. A chiller 88, as shown, is typically provided for the distillate (leaching solution) to condense and collect for reuse. The metals recovered from the distillation station 86 may then be further processed for purification. With reference to FIG. 6, one or more pumps, such as pump 75, may be used to re-circulate leaching solution 70 through any or all of the components of the exemplary metal recovery system 50.

[0042] Referring again to FIG. 6, column supports 62 comprise three support members, each at varying heights, to allow each column 60 to stand at a desired angle. Leaching metal at an angle rather than vertically or horizontally through column 60, is advantageous in that the angle sufficiently slows the flow of the leaching solution 70 through column 60 to allow longer exposure of the metal-containing particulate material 68 to the leaching solution 70, thereby increasing leaching (extraction) yield. In addition, the electric current of the present invention flowing through column(s) 60 during the leaching process increases the amount of metal leached from the particulate material 68. Accordingly, electrically charged leach columns 60 positioned at angles improve recovery and provide greater yields of metal separation per unit time. Recovery system 50 is also efficient from the perspective of the amounts of leaching solution 70 used in recovering the metal. The desired angle is chosen by the user depending on the length of column 60, the desired metal(s) to be separated, and the physical and chemical characteristics of the metal-containing particulate materials 68 and inert materials 76. Also, factoring into the rate of leaching is the flow rate of the leaching solution 70 through column 60. With gravity as the only force influencing the flow of leaching solution 70, as column 60 approaches the horizontal position, the flow rate decreases and longer times are generally required for satisfactory metal separation. The viscosity and texture of the leaching solution 70 may further retard flow rate. To this end, the flow rate may be enhanced through the use of a pump to accelerate the leaching solution 70 through each column 60.

[0043]FIGS. 6 and 7 illustrate one exemplary system having apparatus associated therewith. However, the invention is not so limited, and other components of metal-recovery systems may be utilized. For instance, control devices for controlling the distillation and recovery of the leaching solution, a chiller, and/or a pump (where desired) for regulating the flow rate of the leaching solution through the column, are desirable. In addition, sand shakers, leachate pumps, washing and drying stations and the like are also desirable components.

[0044] The metal recovery system of the present invention is particularly useful for the collection and recovery of platinum, and particularly for the recovery of platinum present in coatings on jet engine components. As discussed in the background section, these coatings typically contain small quantities of platinum between about 1-1.5 gms per coating. Prior art leaching methods have yielded leaching solutions containing platinum at a concentration less than 0.5%. Accordingly, these recovery techniques have not been sufficiently effective to warrant recovery of the platinum. The present invention provides an improved platinum recovery system, thereby allowing the platinum to be recovered and reused in an efficient and cost effective manner. To recover platinum from engine components, the coating on the component is first stripped from the surface. Stripping may be accomplished by traditional techniques used in this art. For example, the coating may initially be exposed to a solution or mixture containing corrosive acids including hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, hydrofluoric acid, and the like, used to dissolve or strip the coating from the surface. A major portion of the coating generally remains on the surface as a collection of residue or smut for removal therefrom. However, small amounts of desirable metal is present in the solution used to strip the surface. This portion may be recovered by simply filtering the solution, and incinerating the filter for recovery of the metal. Other conventional techniques for recovering metal from the filter may be used. The residue containing a major portion of the metal may be removed by conventional, known methods, such as by grit blasting, water/wet blasting techniques, vibratory finishing techniques, hand brushing, scrubbing, and the like. Advantageously, a grit blasting technique may be used where the sand particles are aluminum oxide or silicon dioxide particles having a mesh size of about 240. Other sand of varying mesh size are also suitable for removal of the coating residue. Beads, such as Ballotini beads or silicon dioxide glass beads may be used as the grit for blasting and removing the residue. The residue removed generally contains a majority of the platinum contained in the original platinum-containing coating, but in a concentration range of about 1-5% by weight of the coating residue.

[0045] The residual coating or platinum-containing smut removed from the component surface is mixed in with the grit blasting sand forming a mixture which may be placed within the leach column(s) of the present invention. As discussed herein, and with reference FIGS. 6 and 7, the exemplary metal recovery system 50 may be used to leach platinum from such a platinum-containing particulate material 68 (the residue or smut mixture) removed from jet engine components. To this end, the platinum-containing particulate material 68 is exposed to leaching solution 70, which may be a highly corrosive acidic solution containing various mineral acids at various concentration ratios. For example, hydrochloric acid, hydrofluoric acid, nitric acid, phosphoric acid, sulfuric acid, sulfamic acid, citric acid, oxalic acid, per-acid or peroxide, and combinations thereof, may be used to leach the platinum from the platinum-containing grit blasted particulate material 68. The sand (grit) used for the grit blasting serves as the inert material 76 and comprises a significant proportion of the mixture of sand 76/particulate material 68 such that the mixture may be used to fill the entire column 60 without the need to pre-pack column(s) 60 with additional inert material 76. Charging the column 60 with electricity to flow from the first electrode (not shown) through the acid solution (electrolyte) and sand 76 to the second electrode (not shown) allows the platinum to be leached and recovered in higher percentages and in higher quantities than methods of the prior art. This platinum, once recovered, may be further processed for recovery of pure platinum which may be recycled for further coating purposes.

[0046] By virtue of the foregoing, there is thus provided a leach column, a metal recovery system, and a leaching method by which metals, present in small quantities in a particulate material or mixture may be separated and recovered. The present leaching method yields quantities of desirable metals per unit time greater than leaching methods of the prior art. The electrically charged leach column provides the enhanced recovery. Passage of a current through the leach column during the leaching process enhances the leaching strength of the leaching solution to increase metal recovery over time. In this fashion, desirable metals present in small amounts in metal-containing materials, such as platinum in platinum-containing residues removed from jet engine components, may be effectively and efficiently recovered utilizing the leaching column and methods of the present invention. The metal recovery system thus provides a cost savings to the user, manufacturer, and/or final consumer.

[0047] While the present invention has been illustrated by the description of embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, while the present invention has been described with particular attention to the recovery of platinum from jet engine component coatings, the invention is not so limited, and may be utilized for the recovery of any desirable metal based on the charge in the leaching column, which metal may be present in small amounts in a mixture. For example, the present metal recovery system may be used for the recovery of other metals such as gold, silver, iron, zinc, nickel, cobalt, cadmium, titanium, aluminum, molybdenum, tungsten, tantalum, hafnium, arsenic, niobium, zirconium, rhenium, ytterium, and the like, by utilizing the electrically-charged leaching column of the present invention. In addition, while the exemplary leaching column as shown is cylindrical in shape, the column may be any shape as desired and may comprise any material that is suitable for the purposes of reclaiming the desired metal and is non-reactive to the leaching solution used in the reclamation process. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of Applicant's general inventive concept. 

What is claimed is:
 1. A leach column comprising: a tube adapted to support therein a mixture containing a metal to be leached and adapted to pass a leaching solution therethrough; and a pair of electrical connections at spaced apart locations along the tube and configured to induce an electric current therebetween in the presence of the leaching solution.
 2. The column of claim 1 wherein the pair of electrical connections comprise a first electrode and a second electrode adapted to pass an electric current therebetween.
 3. The column of claim 2 wherein the first electrode and the second electrode are positioned at opposing ends of the leach column.
 4. The column of claim 2 wherein the first electrode and the second electrode comprise a material selected from the group consisting of graphite, nickel-base alloys, chromium-based alloys, nickel-chromium-molybdenum alloys, platinum, platinized titanium, niobium expanded mesh coated with platinum, and palladium.
 5. The column of claim 2 wherein the first electrode and the second electrode comprise graphite.
 6. A system for leaching platinum from a platinum-containing particulate mixture, the system comprising: a leach column adapted to support the platinum-containing particulate mixture therein and to pass a leaching solution therethrough; a leaching solution for passage through the platinum-containing particulate mixture, the leaching solution comprising at least one acid capable of leaching platinum from the particulate mixture; and a pair of electrical connections at spaced apart locations along the column and configured to induce an electric current between the electrical connections in the presence of the leaching solution.
 7. The system of claim 6 wherein the pair of electrical connections comprise a first electrode and a second electrode adapted to induce an electric current therebetween.
 8. The system of claim 7 wherein the first electrode and the second-electrode are positioned at opposing ends of the leach column.
 9. The system of claim 7 wherein the first electrode and the second electrode comprise a material selected from the group consisting of graphite, nickel-base alloys, chromium-based alloys, nickel-chromium-molybdenum alloys, and palladium.
 10. The system of claim 7 wherein the first electrode and the second electrode comprise graphite.
 11. The system of claim 6 wherein the platinum containing particulate mixture comprises platinum-containing particles and inert particles.
 12. The system of claim 11 wherein the inert particles are sand.
 13. A system for recovering platinum comprising: a platinum-containing particulate mixture comprising platinum-containing particles and inert particles; a leaching solution adapted to leach platinum from the platinum-containing particulate mixture; at least one leach column adapted to support therein the platinum-containing particulate mixture and adapted to pass the leaching solution therethrough; a first electrode and a second electrode spaced apart within each column, the first and second electrodes configured to induce an electric current therebetween in the presence of the leaching solution; and an external source of electricity for inducing the electric current in each column.
 14. The system of claim 13 further comprising a reservoir for containment of the leaching solution.
 15. The system of claim 13 further comprising a tank for collection of leachate formed from the leaching solution after passage through the platinum-containing particulate mixture.
 16. The system of claim 13 further comprising a distillation apparatus for recovery of the leaching solution from the leachate.
 17. The system of claim 13 further comprising a collection box for containment of the inert particles.
 18. The system of claim 13 wherein the external source of electricity is a generator.
 19. The system of claim 13 wherein the leaching solution comprises one or more acids selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, phosphoric acid, sulfamic acid, citric acid, oxalic acid, and peroxide.
 20. A method of leaching a metal from a metal-containing particulate mixture comprising: placing a metal-containing particulate mixture within a leach column; passing a leaching solution through the particulate mixture within the column whereby to leach the metal from the particulate mixture; and inducing an electric current through the leaching solution while leaching the metal from the particulate mixture.
 21. The method of claim 20 wherein the metal leached from the metal-containing particulate mixture is selected from the group consisting of platinum, gold, silver, palladium, nickel, copper, zinc, cadmium, rhodium, and combinations thereof.
 22. The method of claim 20 further comprising positioning a pair of electrical connections at spaced apart locations within the leach column for inducing the electrical current therein.
 23. The method of claim 22 further comprising connecting the pair of electrical connections to an external source of electricity for inducing the electric current.
 24. The method of claim 23 wherein the pair of electrical connections comprise a first electrode and a second electrode, and inducing the electric current is by passing electricity between the first and second electrodes in the presence of the leaching solution.
 25. The method of claim 24 wherein the first and second electrodes are positioned at opposing ends of the leach column.
 26. The method of claim 24 wherein the first electrode and the second electrode comprise a material selected from the group consisting of graphite, nickel-base alloys, chromium-based alloys, nickel-chromium-molybdenum alloys, platinum, platinized titanium, niobium expanded mesh coated with platinum, and palladium.
 27. The method of claim 24 wherein the first electrode and the second electrode comprise graphite.
 28. The method of claim 20 wherein the leaching solution comprises one or more acids selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, phosphoric acid, sulfamic acid, citric acid, oxalic acid, and peroxide.
 29. The method of claim 20 further comprising removing a metal-containing coating from a jet engine component to produce metal-containing particles that form at least a portion of the metal-containing particulate mixture for placement within the leach column.
 30. The method of claim 20 wherein the metal leached from the metal-containing particulate mixture is platinum.
 31. A method of recovering platinum from a platinum-containing coating on a jet engine component, the method comprising: exposing the jet engine component to a stripping solution whereby a platinum-containing residue is formed on a surface of the component; removing the platinum-containing residue from the surface of the component; placing the platinum-containing residue within a leach column; passing a leaching solution through the platinum-containing residue within the column; and inducing an electric current through the leaching solution while leaching platinum from the platinum-containing residue.
 32. The method of claim 31 wherein removing the platinum-containing residue from the component surface comprises blasting the component with grit, and wherein the grit is placed in the leach column with the particulate-containing residue removed thereby.
 33. The method of claim 31 wherein the grit is selected from the group consisting of aluminum oxide and silicon dioxide.
 34. The method of claim 31 wherein the grit is beads selected from the group consisting of ballotini beads and silicon dioxide glass beads.
 35. The method of claim 31 wherein the stripping solution comprises one or more acids selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, and phosphoric acid.
 36. The method of claim 31 wherein the leaching solution comprises one or more acids selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, phosphoric acid, sulfamic acid, citric acid, oxalic acid, and peroxide.
 37. The method of claim 31 further comprising providing a pair of electrical connections at spaced apart locations within the leach column for inducing the electrical current therein.
 38. The method of claim 37 further comprising attaching the electrical connections to an external source of electricity.
 39. The method of claim 37 wherein the pair of electrical connections comprise a first electrode and a second electrode, and inducing the electric current is by passing electricity between the first and second electrodes in the presence of the leaching solution.
 40. The method of claim 39 wherein the first electrode and the second electrode are positioned at opposing ends of the leach column.
 41. The method of claim 39 wherein the first electrode and the second electrode comprise a material selected from the group consisting of graphite, nickel-base alloys, chromium-based alloys, nickel-chromium-molybdenum alloys, and palladium.
 42. The method of claim 39 wherein the first electrode and the second electrode comprise graphite.
 43. The method of claim 32 further comprising collecting the grit removed from the leach column during leaching for reclassification and reuse.
 44. The method of claim 43 further comprising washing the collected grit for separation from the leaching solution.
 45. The method of claim 44 further comprising drying the washed grit for reclassification and reuse.
 46. A method of recovering platinum from a platinum-containing coating on a jet engine component, the method comprising: stripping the platinum-containing coating on the jet engine component with a stripping solution comprising one or more acids selected from the group consisting of hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, and phosphoric acid, whereby a platinum-containing residue is formed on a surface of the component; blasting the surface of the component with grit to remove the platinum-containing residue and thereby form a mixture of grit and the platinum-containing residue; placing the mixture within a leach column; passing a leaching solution through the mixture within the column; and inducing an electric current through the leaching solution while leaching platinum from the mixture.
 47. The method of claim 46 wherein the grit comprises a material selected from the group consisting of aluminum oxide and silicon dioxide.
 48. The method of claim 46 wherein the grit is selected from the group consisting of ballotini beads and silicon dioxide glass beads.
 49. The method of claim 46 further comprising positioning a first electrode and a second electrode at spaced apart locations within the column, the first and second electrodes configured to induce the electric current therebetween in the presence of the leaching solution.
 50. The method of claim 49 wherein the first electrode and the second electrode comprise a material selected from the group consisting of graphite, nickel-base alloys, chromium-based alloys, nickel-chromium-molybdenum alloys, and palladium. 